Rollout for the Games, whose opening ceremony is July 27, has gone fairly smoothly, with stadiums' construction finished or nearly so. But price has grown sharply, with security cost a factor.

LONDON — As a longtime Londoner but not much of a sports fan, Elaine Potter has cast a mostly skeptical eye over the Olympics and all the related fuss — the omnipresent marketing, the endless media coverage.

But gazing out over the shiny new stadium in London's East End for the first time this week, with a friend who dragged her to go see it, Potter admitted to some second thoughts.

"It's looking good," said Potter, 56, who's tempering her opinion of the Games as one big headache. "I think it's quite good for London and for making London known."

On Wednesday, 100 days before the July 27 opening ceremony, the countdown begins in earnest for the world's biggest sporting event — and for the organizers charged not just with making sure venues and volunteers are ready but also with getting residents like Potter to feel excited about it and to change the way they live and work, at least temporarily.

So far, except for a few glitches, the Olympic rollout has gone fairly smoothly, a showcase of British know-how and efficiency, officials say. Construction on several new stadiums is finished or nearly there, an extensive transportation plan is being refined, a massive security operation is underway and tickets have sold like hot cakes.

But there's little room to take anything for granted here in one of the world's most densely packed and gridlocked cities, a magnet for tourists and terrorists alike.

And London's status as an expensive destination has been amply borne out by the Games' price tag, which started at less than $4 billion but has been marked up several times to the latest figure, a whopping $15 billion. Critics say that could rise to as much as $21 billion in the end, which, though only half of what China spent on the 2008 Olympics in Beijing, is an immense sum in a time of the harshest government cutbacks in a generation.

Recently, a parliamentary report warned that the Olympic budget was "finely balanced," in danger of tipping into the red if organizers weren't vigilant.

The delicate financial situation, lawmakers said, is due almost entirely to a single factor: security.

A year and a half ago, the organizing committee confidently signed a contract with a security company worth about $136 million. But the estimated cost of keeping the Olympics safe has since skyrocketed to $875 million, in part to hire twice the number of guards originally thought necessary to patrol venues.

Mindful that the Games and London make a juicy "two for one" target for terrorists, the government has pledged 7,500 British troops to back up the guards at the various arenas. That has led to fears of an overly intrusive security presence in a city already blanketed by surveillance cameras.

"Our first priority must be to keep people safe, and we will be mobilizing every aspect of our security infrastructure," Prime Minister David Cameron said last month. "But it will be done in a way that is sensitive to the spirit of the Games."

The ballooning security costs have put organizers on notice, though some Londoners snorted in derision when Sebastian Coe, the former gold medalist who runs the planning committee, said in December that the Olympics would "be living hand-to-mouth."

The event's eye-popping budget is a far cry from the last time the British capital hosted the world's top athletes, 64 years ago.

In 1948, the country still hadn't emerged from the shadow of war, and the depleted treasury meant it was forced to put on the "Austerity Olympics," with frugality as the watchword. Not a single new stadium was built, nor an Olympic village. Some national teams even brought their own food; the French, naturally, carted in their own wine.

Yet the event was a triumph — it even turned a profit — and gave a major boost to British confidence, a success today's organizers are fervently hoping to replicate.

In many ways, more thought has gone into how to capitalize on the Olympics' high profile, afterglow and infrastructure than any previous Games. The bid committee's plans for the Olympic "legacy," particularly how the new stadiums will be put to use after all the visitors go home, were key to winning the hosting contest back in 2005.

Many of the new venues are concentrated in East London, a deprived area once described as an "urban desert" littered with ramshackle housing projects and pools of fetid water. Because of the Olympics, a huge shopping mall, an athletes' village that will be turned into apartments, a new school and other improvements have sprung up or are being planned.

The 80,000-seat main stadium sits in the largest green space created in London in centuries. Officials like to point out that the Olympic Park was built ahead of schedule and under budget.